Phosphorus Removal Structures
Phosphorus removal structures (PRS) are engineered landscape-scale filters that contain P sorption materials (PSMs) which have a high affinity for dissolved P (DP), thereby removing dissolved P from flowing surface or subsurface drainage.
Soils with excessive soil test P concentrations can behave as long-term sources of DP (“hot spots”) to surface water bodies, even if other BMPs are implemented for reducing sediment and particulate P transport. After a soil becomes built up with excessive soil P, it will require many years of soil P-drawdown to decrease soil P levels. In the meantime, no other BMP will stop DP loss without eliminating water from leaving a field.
The PRS targets DP for the additional reason in that DP causes more damage to surface waters compared to particulate P (i.e. P bound to sediment).
How Does This Practice Work?
The PRS can vary in appearance and application, but all PRS possess four basic characteristics:
- Contains solid media with high affinity for P, commonly known as a “P sorption material”, or PSM
- PSM is contained and placed in a hydrologically active area with high DP concentrations.
- High DP water is able to flow through the contained PSM at a sufficient rate.
- The PSM is able to be removed and replaced after it is no longer effective.
Some PSMs are manufactured while others are by-products. Most PSMs produced as by-products from industry must first be screened for trace metals before use in a P removal structure. Regular steel slag should only be used for treating surface water, not subsurface drainage, since bicarbonate contained in tile drainage will cause slag to clog.
Choosing a suitable location
In order to qualify as a potential site for construction of a P removal structure, a site must possess:
- Flow convergence to a point where water can be directed into a structure, or the ability to manipulate the landscape
- At least 0.2 ppm dissolved P (DP) in water, or source soil with at least 200 lb/acre Mehlich-3 P.
- Hydraulic head required to “push” water through structure: function of elevation change or drainage ditch depth
- Sufficient space to accommodate PSM
Keep in mind that the P removal structure is ideally targeted to capture DP from legacy P soils, not recently applied manure and fertilizer in which less expensive 4R principles can be utilized.
Designing a PRS
Several inputs and target goals are required for designing a site specific structure. The freely available P-Trap software (P transport reduction app) can be used to quickly design a structure with any available PSM. The software and other information is found at: https://www.ars.usda.gov/nserl/ptrap
Structures are constructed based on a desired P removal goal and lifetime in accordance with site conditions (flow rate, annual flow volume, and DP concentration). For example, a 10yr-40% goal means that the structure is designed for removing 40% of all the DP that flows into a structure over 10 years. For details on P removal structures and how to gather inputs, see Penn, C.J., and J.M. Bowen (2017). A series of training videos on designing PRS are available from the American Society of Agronomy and American Society of Agricultural and Biological Engineers. Contact your local NRCS conservation district about Standard # 782 if you are interested in having a structure constructed.
Size of PRS
Because PRS are sized according to the site conditions and P removal goals, small structures are not effective with regard to removing appreciable P loads. This is due to the fact that small units, those that contain pounds and not tons of PSMs, do not have the capacity to remove appreciable P or handle a significant flow rate. For example, cartridge filters, buckets, and socks.
Cost of Implementing the Practice
The only other conservation practice that can truly remove soil P is drawdown via plant uptake and harvest. Although necessary, it requires many years to reduce DP concentrations. During that long draw-down period, P removal structures can trap P in drainage water (see figure below).
Cost varies from $3 to $20K, depending on the size of the field, DP loading, site conditions, choice of PSM, and P removal goals. P-TRAP is useful for estimating size and therefore cost. To date, the most cost effective PSM in most situations is metal shavings mixed with gravel.
Operation and Maintenance
Implementation of other BMPs for preventing sediment deposition into a PRS is necessary to prevent premature clogging. Spent PSMs can be disposed by a variety of ways, depending on the material. For example, gravel-steel shavings mixtures and steel slag can be applied to farm road surfaces since the P is tied up with little solubility. Other spent PSMs can be land applied to soils with regular P levels, although there will be no P fertilization benefit.
Penn, C. J., & Bowen, J. M. (2017). Design and construction of phosphorus removal structures for improving water quality. Springer.
Penn, C. J., Frankenberger, J., & Livingston, S. (2021). Introduction to P‐TRAP software for designing phosphorus removal structures. Agricultural & Environmental Letters, 6(1), e20043.
Penn, C., Chagas, I., Klimeski, A., & Lyngsie, G. (2017). A review of phosphorus removal structures: How to assess and compare their performance. Water, 9(8), 583.
USDA-ARS National Soil Erosion Research Laboratory
Editing and Design
NC State University
University of Tennessee
Penn, C. 2023. Phosphorus Removal Structure. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/phosphorus-removal-structures/